1. Field of the Invention
The present invention relates to a light emitting device which emits yellowish light, for example, a light emitting device used as a backlight for liquid crystal display and other displays.
2. Description of the Related Art
There is demand for a light emitting device which emits yellow light for such applications as signals, illuminations and signboards.
The light source for a Signboard is currently a fluorescent lamp which is inexpensive and high emission efficiency. However, the fluorescent lamp has short life which requires frequent replacements and a high running cost. The fluorescent lamp also has limitations on the size and configuration thereof, which make it impossible to design the signboard at will or manufacture it at a low cost. Also from the view point of environment, it is called for to use a light source which does not involve mercury. With this background, much attention is focused on a light emitting device based on a light emitting element.
A light emitting device using a light emitting element is small in size, high in the power efficiency and emits light with bright color. Also the light emitting element is made of a semiconductor and is therefore free from burnout. Moreover, the light emitting element is far smaller than the fluorescent lamp, and therefore allows it to make a signboard in a free design when used as the light source thereof. The light emitting element also has such remarkable features as excellent startup characteristic, high resistance to vibration and robustness to endure a lot of on/off operation cycles. Because of these advantageous properties of the light emitting element, light emitting devices which employ semiconductor light emitting elements such as light emitting diode (hereinafter referred to as LED) and laser diodes (hereinafter referred to as LD) are used as the light source in various applications.
Prior art technologies related to the light emitting device using the light emitting element can be found in Japanese Unexamined Patent Publication (Kokai) No. 2002-237617 and National Publication of Translated Version (Kohyo) No. 2003-505583. For example, Japanese Unexamined Patent Publication (Kokai) No. 2002-237617 discloses a light emitting device for emitting yellow light which uses a yellow color LED. This yellow color LED has double heterojunction structure wherein an active layer formed from InGaAlP-based material is sandwiched by cladding layers formed from InGaAlP-based material, the structure being formed on a GaP substrate.
National Publication of Translated Version (Kohyo) No. 2003-505583 discloses, not a light emitting device that emits yellow light as described above, but a light emitting device that emits white light by combining a blue color LED that emits blue light and a fluorescent material which emits yellow light or yellowish green light. The fluorescent material has the garnet structure A3B5O12 activated with Ce. First component A of this fluorescent material includes at least one element selected from among a group consisting of Y, Lu, Se, La, Gd and Sm, and second component B includes at least one element selected from among a group consisting of Al, Ga and In. The fluorescent material has garnet structure wherein at least a part of the first component A is replaced with Tb. The type A3B5O12 fluorescent material is excited by blue light emitted by the LED and emits yellow light. Thus this light emitting device produces white light through mixing of the blue light emitted by the LED and the yellow light emitted by the type A3B5O12 fluorescent material. This light emitting device is widely used because of low power consumption and easy control of the driving of the LED.
The light emitting device which uses yellow color LED currently available is not capable of changing the color tone of the yellow light. This is because there are some variations among the products, but the color tone of the light emitted by the LED is substantially constant. When it is desired to make a yellow color LED emitting light of a particular color tone, the active layer or the like may be doped with an impurity. However, it is not easy to control the color tone by this method. Moreover, there has been no yellow color LED developed for practical use which can emit light with a level of luminance high enough for use in a signboard.
A white light emitting device which combines a blue color LED and a type A3B5O12 fluorescent material can work as a yellow light emitting device by increasing the content of the type A3B5O12 fluorescent material. However, since the light emitting device of this type mixes blue light and yellow light, the resultant light tends to have whitish yellow color. In order to obtain yellow light of a desired color tone, it is necessary to have a large amount of the type A3B5O12 fluorescent material disposed in a very high density in the vicinity of the LED. A large amount of the type A3B5O12 fluorescent material disposed in a very high density causes such a problem that yellow light emitted by the type A3B5O12 fluorescent material is blocked by other particles of the type A3B5O12 fluorescent material and it becomes difficult to take out light from the light emitting device in the output direction. As a result, although the light emitting device emits yellow light, the luminance of emission is low and yellow light emitting device of high luminance cannot be obtained.